How to match a separate dome mesh for an eye to an irregular eye hole perimeter?
Your shape is a bit hard to work as it is already high in polygons, you should not work with high-poly as long as you are not satisfied with the whole shape in my opinion (and even afterwards, keep it low-poly, use a Subdivision Surface modifier to smooth, and only apply it if necessary), but, starting with what you're sharing, what you could do is duplicate the outer edge loop, simplify it (header menu &gt; Select &gt; Checker Deselect, then X &gt; Dissolve vertices):

Then extrude inwards and fill:

Put it on the head, make some adjustments:

If you want to smooth it, give it a Subdivision Surface modifier (3 subdivisions) and apply:

Here is what it gives: